1. Field of the Invention
The present invention relates to an apparatus and method for measuring chromatic dispersion in an optical fiber.
2. Description of the Related Art
Single-mode optical fibers in which chromatic dispersion is zero around the wavelength of 1.3 μm and is approximately 17 ps/nm/km around the wavelength of 1.55 μm, dispersion-compensating optical fibers in which chromatic dispersion is negative around the wavelength of 1.55 μm, and dispersion-shifted optical fibers in which the zero dispersion wavelength is shifted from the wavelength of 1.3 μm toward the longer wavelength side have been used thus far as optical fiber transmission media in optical communication systems. A dispersion-managed optical fiber is also under consideration in which chromatic dispersion varies, having an opposite sign alternately in the longitudinal direction at the wavelength of 1.55. Known methods for measuring chromatic dispersion in an optical fiber are a pulse delay method, a phase shift method, and an interferometric method (Giovanni Cancellieri, Single-Mode Optical Fiber Measurement: Characterization and Sensing (Artech House, Boston, 1993), chap 3, pp. 139–144).
If chromatic dispersion in an optical fiber can be measured by in-line measurement during a manufacturing process in which the optical fiber is drawn from an optical fiber preform, it is possible to make the optical fiber to have a designed chromatic dispersion by controlling the drawing conditions (for example, the drawing speed, the optical-fiber-preform feeding speed, and the drawing-furnace temperature) on the basis of the result of such measurement. However, none of the above methods can be applied to the in-line measurement.
That is, in the pulse delay method, an optical fiber having a length of 0.5 km or more is necessary for measurement. In the phase shift method, it is necessary to measure the amount of phase shift of each of the optical signals having different wavelengths that are made to enter an optical fiber to be measured, and this increases the measurement time. In the interferometric method, it is necessary to measure interference signals while adjusting the difference in optical path length between two optical paths in an interferometer, and thus the measurement consumes considerable time. For this reason, it is difficult to perform in-line measurement using the above methods.